In a wireless communication system, a control signal is transmitted from a base station to a user equipment, and the user equipment controls communication in accordance with the control signal. For example, in the 3rd Generation Partnership Project (3GPP), a physical downlink control channel (PDCCH) is defined as a communications channel for transmitting a control signal from a base station to a user equipment.
The PDCCH allows each control signal to be transmitted to a corresponding one of a plurality of user equipments. That is, the resources of the PDCCH are allocated to a plurality of user equipments. Here, a base station allocates one or more control channel elements (CCEs) as the resources of the PDCCH to each user equipment.
Based on a channel quality indicator (CQI) notified from a user equipment, a base station determines the number of CCEs to be allocated to the user equipment. That is, based on a channel quality indicator (CQI) notified from a user equipment, a base station determines the PDCCH format corresponding to the user equipment. The CQI is an index indicating the quality of communication between a base station and a user equipment, and is measured in each user equipment.
A base station determines the PDCCH format (that is, the number of CCEs) corresponding to each user equipment, based on the below relationships. Note that characters a, b, and c in the below relationships each represent communication quality. Communication quality of a is better than communication quality of b, and the communication quality of b is better than communication quality of c. The relationships between conditional expressions for the CQI and the PDCCH formats are as follows: a≦CQI: PDCCH format #0 (one CCE), b≦CQI<a: PDCCH format #1 (two CCEs), c≦CQI<b: PDCCH format #2 (four CCEs), and CQI<c: PDCCH format #3 (eight CCEs).
For example, when the communication quality between a base station and a user equipment (that is, the CQI measured by the user equipment) s higher than “a”, format #0 that uses only one CCE is selected. When the communication quality between a base station and a user equipment is lower than “c”, format #3 that uses eight CCEs is selected.
As such, when the communication quality between a base station and a user equipment is good, the base station selects a PDCCH format using a small number of CCEs. Here, when the communication quality is good, a signal may be transmitted using a modulation scheme in which the amount of information per symbol is large. Consequently, in this case, a control signal is transmitted with a few resources.
In contrast to this, when the communication quality between a base station and a user equipment is not good, the base station selects a PDCCH format using a large number of CCEs. Here, when the communication quality is poor, it is preferable, in order to constrain transmission errors, that a modulation scheme in which the amount of information per symbol is small is used. Accordingly, in this case, since a control signal is transmitted at a low transmission rate, the format that uses many resources is selected.
In order to increase the number of user equipments that are allowed to be located in the coverage area of a base station, it is preferable that the resources of the PDCCH allocated to each user equipment are decreased. When the resources of PDCCH allocated to each user equipment are excessively few, however, the error rate of control signals transmitted via the PDCCH is degraded, and may, in some cases, lead to disconnection of communication between the base station and the user equipment. Thus, in a wireless communication system that determines a PDCCH format based on communication quality between a base station and a user equipment (that is, a CQI measured by the user equipment), it is preferable that thresholds (that is, the above mentioned characters a, b, and c) for selecting a PDCCH format are determined in consideration of both of efficient utilization of radio resources and constraint on transmission errors. Then, in one embodiment, the thresholds for selecting a PDCCH format are determined based on measurements or a simulation at the time of construction of a wireless communication system, for example.
Note that a reception format decoding method mentioned hereinafter has been proposed as a related art technique. In this method, a frame is transmitted from a base station to a user equipment in a frame format selected from a plurality of types in accordance with the propagation path between the base station and the user equipment. When decoding the frame, the user equipment determines the order of decoding of frame formats in accordance with the state of the propagation path environment (see, Japanese Laid-open Patent Publication No. 2010-130121).